Cre is a site-specific recombinase that performs double stranded cleavage and rejoining at particular 34 bp sequence lox. Cre is widely used to catalyze chromosome rearrangements and transgene integration in living cells. We have been pursuing structure of an inactive Cre recombinase mutant bound to its target DNA sequence, to determine the specificity Cre/DNA interactions in the first step of its reaction cycle. We have a preliminary 3.0 E structure in which Cre assumes a novel oligomerization state and will obtain a higher resolution data set at SSRL. We will also obtain native and derivative data on another crystal form, which appears to be the correct precleavage complex and diffracts to 3.5 E at home.